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Applications of Newton's laws

  1. Jan 30, 2015 #1
    1. The problem statement, all variables and given/known data
    You've attached a bungee cord to a wagon and are using it to pull your little sister while you take her for a jaunt. The bungee's unstretched length is 1.3m and you happen to know that your little sister weighs 220N and the wagon weighs 75N. Crossing a street, you accelerate from rest to your normal walking speed of 1.5m/s in 2.0s, and you notice that while you are accelerating, the bungees length increases to about 2.0 cm. what is the force constant of the bungee cord, assuming it obeys hooke's law?

    2. Relevant equations
    f=ma
    fs=kx

    3. The attempt at a solution

    Ok, so I am quite stumped with this question, i know you should find the total force of the little sister and wagon , 220+75, but i do not know if i should keep it as a force or as a total mass in kg. And the difference in spring length 2.0-1.3=.7m . I am just unaware if i bring kinematic forumlas into this and i am quite unaware of what to find because i am getting the wrong answer when looking for "k"

    Any guidance is appreciated. Thanks
     
  2. jcsd
  3. Jan 30, 2015 #2

    mfb

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    Staff: Mentor

    The problem has a serious flaw, but we can ignore that for now.

    I guess the 2.0 cm mean 2.0 m. The weight is a force downwards - as weight it won't help you much as your bungee cord is horizontal. Finding the mass is a good start.
    What is the force needed to stretch the bungee cord? What does that force act on?
     
  4. Jan 30, 2015 #3
    yeah its 2.0 m . long day lol . ok , so it would be total mass over gravity, giving you; 295/9.80= 30.102kg ? acceleration is 1.5/2 = 0.75 m/s^2. I dont get what you are asking in the second part though.
     
  5. Jan 30, 2015 #4
    cant believe i had that brain fart. thanks !
     
  6. Jan 30, 2015 #5

    mfb

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    Let me rephrase that: Why does the wagon accelerate, and which force (in N) comes from where?

    Edit: Okay, solved?
     
  7. Jan 30, 2015 #6
    from the pull? and i multiplied the mass and acceleration and divided by the .7m difference in the spring?
     
  8. Jan 31, 2015 #7

    mfb

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    Staff: Mentor

    Looks fine.
     
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